On Radio-Frequency Based Detection of High-Frequency CirculatingBearing Current Flow

Annette Muetze Ville Niskanen Jero AholaGraz University of Technology Lappeenranta University Lappeenranta University

8010 Graz, Austria 53850 Lappeenranta, Finland 53850 Lappeenranta, Finlandmuetze@tugraz.at ville.niskanen@lut.fi jero.ahola@lut.fi

Abstract—The possibility of bearing damage caused byinverter-induced bearing currents in modern variable-speed drivesystems has been well recognised today. Further research isneeded to develop appropriate non-destructive methods for de-tection and monitoring of such currents. A radio-frequency basednon-destructive method has been applied to detect dischargebearing currents. The method is understood to work on theenergy radiated during the bearing discharge event. Recentresearch has shown that the method is also to some extentapplicable to high-frequency circulating bearing currents. Thesefindings will be presented in this paper. Furthermore, the analysisand understanding of the applicability of the method to detectsuch currents also contributes to further understanding of theelectric characteristics of the bearing, notably the moment thecurrent conduction begins.

NOMENCLATURECM Common mode.HF High frequency.RF Radio frequency.NDE Nondrive-end.DE Drive-end.PE Protective earth.

I. MOTIVATION

The possibility of bearing damage caused by inverter-induced bearing currents in modern variable-speed drive sys-tems has been well recognised today. Different authors havedescribed the cause-and-effect chains, allowing the selectionof appropriate mitigation techniques (e.g. [1]–[7]). Notably,distinction between (a) discharge bearing currents, that aredirectly related to the high-frequency (HF) common-mode(CM) voltage, and (b) HF circulating current that are causedthrough inductive coupling by the HF stator CM current andthat are thus more prevalent with machines with larger framesizes, is important.

In general, intrusive techniques are applied to measure suchbearing currents: Commonly, an electrically insulating layer isintroduced into the current path. This electrical insulation isthen shortened with a small wire, and the current flow throughthe wire measured. Such a method is not suitable for wide-spread cost-effective application in the field. Furthermore,the measurement circuit affects the measured currents. Whilemodels to conclude on the current flow in the respective systembefore modification are available (e.g. [8]), they only reflectthe existing understanding and thus have limited applicabilityto enhance the understanding of the current flow mecha-nism. Thus, further research is needed to develop appropriate

non-destructive methods for the detection and monitoring ofinverter-induce bearing currents and to better understand thecurrent-conduction mechanism within the bearing.

A radio-frequency (RF) based non-intrusive method to de-tect discharge bearing currents has been presented and usedto evaluate and further understand the occurrence of dischargecurrents [9]–[11]. The method is based on the understanding ofan electric machine as a spark gap transmitter with some of theenergy stored within the bearing and machine (notably air gap)before the discharge being emitted as an RF signal. Accordingto the conventional understanding of the current flow of HFcirculating currents, the HF voltage in the loop at the originof these currents causes the bearings to not have electricallyinsulating properties, but show ohmic behavior. Based on thiscurrent understanding, such currents cannot be detected witha RF based method, because of the lack of occurrence of adischarge and subsequent release of energy that can be radiatedoutside of the bearing.

We present results from further investigations of the switch-ing instant during which the HF voltage between the two bear-ings increases, the bearing lubrication film cannot maintainelectrically insulating properties, and HF circulating currentsstart to flow. We show that the RF based method is alsoto some extent applicable to detect HF circulating bearingcurrents. The understanding of the applicability of the methodis tightly coupled with further findings on the electric char-acteristics of the bearing, notably the moment the currentconduction begins: We have observed instantaneous capacitivebehavior of the bearings already at low rotational speed anddischarges that can be associated with the subsequent flow ofHF circulating currents. Experimental results with supportingtheoretical considerations are given following short reviews ofthe two HF bearing current mechanisms referred to above, theRF based method, and the test setup.

II. REVIEW OF HF BEARING CURRENTS AND RF BASEDBEARING CURRENT DETECTION

A. Review of HF Bearing Currents

The nonzero HF CM at the output of modern fast-switchinginverters typically changes with every inverter switching in-stant and arrives at the motor terminals with a high dv/dt,where it interacts with the HF machine impedance.

1) Discharge bearing currents result from the stator windingHF CM voltage charging the bearings via a capacitivevoltage divider, and occur–statistically distributed–as

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discharge current pulses (of up to a few amperes) whenthe threshold voltage of the bearings (that depends onthe operating conditions and typically is in the range ofa few up to some tens of volts) is exceeded.

2) HF circulating bearing currents are caused by inductivecoupling through the HF stator CM current (Figs. 1 and2). In contrast to the discharge currents, the circulatingcurrents occur the moment a switching event takes place.The frequencies of these currents is typically in therange of a few hundred kilohertz, with the first half-period of the oscillation sometimes reaching one to twomegahertz. In general, it is understood that the voltage inthe loop driving the HF circulating current leads to thebearing lubricating film being “punctured”, the bearingshows ohmic behavior, and the bearing resistance isso small that it is usually neglected in the proposedequivalent circuits.

stator frame

rotor core

stator winding

stator core

end shield

F0

bearingib

shaft

Fig. 1. Path of HF circulating bearing current [12].

Common mode

current

20 A/Div

Bearing currents

(NDE)

(DE)

5 A/Div

1 ms/Div

Fig. 2. Measured HF circulating bearing currents, induction motor, framesize 400mm, 500 kW rated power, motor speed n = 3000 rpm, bearingtemperature θb ≈ 70°C [12].

B. Review of RF Based Bearing Current Detection

It has been shown that a fraction of the energy releasedduring the discharge of a discharge bearing current is radiatedoutside from the electric motor and can be detected by appro-priate equipment. In this case, the machine itself operates as aspark gap transmitter. The characteristics of the RF radiation(e.g. power, frequency range, radiation pattern) are determinedby the characteristics of the electrical discharge and those ofthe electric machine as the transmitting antenna. The frequencyband of the radiation has been determined to be 90−400MHz;the machine shaft end has been shown to play a key role inthe transmitting characteristics, and the radiated power, eventhough small, is sufficiently large to be detected.

III. TEST SETUP

A. Drive Systems

Two drives with two different power levels and thus framesizes are used for the experiments. Both are 230/400V, 50Hz,∆-connected, 4-pole induction motors operated by three-phase400V, 50Hz inverters. The two motors and inverters arereferred to as motors MA-15 and MB-75 as well as invertersIA-15 and IB-75 respectively. MA-15 is a 160mm framesize 15 kW, MB-75 a 280mm frame size 75 kW machine. Thetwo 400V inverters are rated at 14.8A and 82A respectively.The smaller inverter is operated at 4 kHz (scalar control,constant switching frequency), the larger one at 3 kHz (directtorque control, average switching frequency). Both machinesare grounded through the PE conductor of the motor cableonly. Bearing temperatures during operating were in the rangeof (25 . . . 35)°C.

In order for the bearing currents to be measured, thebearings of the example case machines are insulated towardsthe housing using an electrically insulating layer of 5mmthickness applied around the outer bearing race that was short-ened with a short wire. As discussed above, this technique isintrusive and will slightly alter the HF current flow when com-pared to the unmodified case. Using the techniques presentedin [8], this influence is estimated to reduce the amplitudes ofthe HF circulating bearing currents by (-10 . . . 15)% for MB-75, and by up to 25% for MA-15.

B. Measurement Equipment

The measurement equipment included an EMCO 93148antenna that has a bandwidth of 200MHz to 2GHz, a Tex-tronix TDS7140 oscilloscope with a bandwidth of 1GHz and amaximum sampling rate of 10GS/s, and an RF bandpass filterMini-Circuits BHP100+ with a bandwidth of 90 − 400MHz(input impedance set to 50Ω). The HF bearing currents on thenondrive-end (NDE) and drive-end (DE) sides were measuredwith a Tektronix TCP202 50MHz current and the voltageacross the bearing with a Tektronix P5210 50MHz highvoltage differential probe (Figs. 3 and 4).

C. Types of Tests

Three types of tests are carried out:A. HF circulating bearing current flow “as is”: The electric

machine is operated at low rotational speed and the HFcirculating bearing currents–if flowing–are detected.

B. Generation of increased HF circulating bearing currentflow: The NDE bearing and its insulation are shortenedto decrease the impedance of the path of the circulatingcurrent. This additional measure increases the ampli-tudes of the HF circulating currents and the likelinessfor these to occur with the small motor MA-15 and/orat increased rotational speed.

C. External supply of HF bearing currents using a HamegHM8131-2 15MHz signal generator: The NDE bearinginsulation was left open: In this configuration, anycurrent flow across the NDE bearing is negligible. TheDE bearing was externally supplied with a HF voltage

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Fig. 3. Measurement setup of motor MA-15 (15 kW).

Fig. 4. Measurement setup of motor MB-75 (75 kW).

within the frequency range typical for HF circulatingbearing currents using the HF signal generator.

For all types of tests, the HF bearing currents, voltage, andimpedance, and RF emission/detection properties are analyzed.

IV. RESULTS

A. HF Circulating Bearing Current Flow “As Is”

1) Experimental Results with Motor MA-15: Because ofits small frame size, in line with the scaling laws for inverter-induced bearing currents, only discharge bearing currents

occurred in this first type of tests and thus measurement resultsobtained with this machine are not discussed in this section.

2) Experimental Results with Motor MB-75: With thismachine, HF circulating bearing currents occurred–along withdischarge bearing currents–up to slightly above 200 rpm ro-tational speed. Above, only discharge bearing currents wereobserved. Maximum amplitudes of the HF circulating bearingcurrents reached up to 1.2A for 100 rpm. They decreaseddown to approximately 0.6A for 210 rpm at which speed HFcirculating bearing currents were only randomly observed. Asthe bearing voltages and currents were measured, too, thebearing current type could be verified for any bearing currentsdetected through the non-destructive RF based method.

Conventionally, the HF circulating bearing current has beenunderstood to flow through the bearing that has mainly ohmicbehavior. During such purely ohmic behavior, there would bean energy conversion due to ohmic loss within the bearing, butno energy release as a result of a discharge. However, as shownin Figs. 5 and 6, also such HF circulating bearing currents canbe detected through the RF based method: The measured HFbearing currents through the NDE and DE bearings have thesame waveforms and amplitudes and opposite signs, which is aclear indicator of HF circulating bearing currents. The momentthe currents start to flow, an RF current pulse is detected. Thisimportant finding will be further analyzed below.

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Fig. 5. Motor MB-75 and inverter IB-75: HF circulating bearing currentdetected through non-intrusive RF based method; measured currents, voltagesand RF signals for 3.3Hz supply frequency (corresponding to 100 rpm).

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Fig. 6. Motor MB-75 and inverter IB-75: HF circulating bearing currentdetected through non-intrusive RF based method; measured currents, voltagesand RF signals for 3.3Hz supply frequency (corresponding to 100 rpm).

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3) Analysis: The detected RF pulse indicates that someenergy has been released and radiated outside the machine themoment the individual current has started to flow. A detailedconsideration of the voltages measured across the NDE andthe DE bearings shows a relatively steep voltage rise notablyin the NDE bearing the moment the current starts to flow. Thevoltage and current waveforms are not fully proportional asone would expect for purely ohmic behavior.

We interpret this as follows: The moment the voltage in theloop increases, the bearing lubricating film still has electricallyinsulating properties. A certain voltage is required for the filmto lose its insulating properties. At a certain threshold, thebearing(s) start(s) to conduct. In the case of discharge bearingcurrents, the energy stored across the bearing(s) is releasedin a single moment. In the present case of HF circulatingbearing currents, some of this energy is absorbed during thechange of the electrical properties of the bearing reducingthe bearing impedance further whereby part of it is releasedand radiated through the electric machine antenna structure.With HF bearing currents both bearings are required to haveelectrically conducting properties. Depending on the drive,some CM capacitive coupling might exist additionally. Suchvoltage would add to the differential voltage induced by theCM current generated HF flux, increasing the latter across theone and decreasing it across the other bearing before the HFcirculating bearing current flow begins.

The energy released has been computed from the energystored in the total capacitance (two bearing and rotor-to framecapacitances) (see eqs. (2) and (3) in [10]). The calculatedvalues range between 10 and a few 100 nJ and are well withinthe range of energies released in the case of discharge bearingcurrents. Comparing Figs. 5 and 6, both show the relativelysteep voltage rise the moment the current starts to flow. Thedetected RF pulse is larger for Fig. 6 where the voltage risesfaster and higher. Note that the maximum bearing current islarger in Fig. 5 indicating that the amplitudes of the bearingcurrent and of the RF pulse might not strongly related.

B. Increased HF Circulating Bearing Current Flow1) Experimental Results with Motor MA-15: Some flow of

HF circulating bearing currents in the smaller machine couldbe generated through the decreased impedance of the bearingcurrent path. With this machine, the preliminary findings wereobtained that indicated the applicability of the method andthat led to further research on the larger machine. For thisreason they are also briefly mentioned in this paper. However,these currents are much more rare with the small machine,and the energies released were often found to be so lowthat RF based detection was difficult. They will thus not bediscussed any further. However, we would like to point outthat this limitation does not impede on the practicability ofthe proposed method: With HF circulating bearing currentstypically not occurring with machines with small frame sizes,but if suffering from HF bearing currents, being put at risk dueto discharge bearing currents, there is no need to detect HFcirculating bearing currents with such machines, and detectionof discharge bearing currents through the RF method has beenwell proven.

2) Experimental Results with Motor MB-75: The investi-gations discussed in this section focused on this motor, wherethe HF circulating bearing currents were much more distinctthan with the smaller machine. Maximum amplitudes of theHF circulating bearing currents reached again up to 12A for100 rpm and decreased down to approximately 6A at 300 rpmwith a few currents even to be found at 450 rpm.

Figs. 7 and 8 show RF based detections of HF circulatingbearing currents at 150 and 300 rpm respectively. Again, themeasured HF bearing currents through the NDE and DE bear-ings have the same waveforms and amplitudes and oppositesigns, and an RF current pulse is detected the moment thecurrents start to flow.

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Fig. 7. Motor MB-75 and inverter IB-75, NDE bearing shortened: HFcirculating bearing current detected through non-intrusive RF based method;measured currents, voltages and RF signals for 5Hz supply frequency(corresponding to 150 rpm).

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Fig. 8. Motor MB-75 and inverter IB-75, NDE bearing shortened: HFcirculating bearing current detected through non-intrusive RF based method;measured currents, voltages and RF signals for 10Hz supply frequency(corresponding to 300 rpm).

3) Analysis: The maximum amplitudes of the HF bearingcurrents at very low rotational speed do not increase as theNDE bearing is shortened, indicating that the influence ofthe bearing impedance at this speed, and once HF circulat-ing bearing currents are flowing, is negligible. However, themaximum speed up to which HF circulating bearing currentswere found increased almost by a factor of 2, supportingthe understanding that the role of the bearing impedanceincreases with increasing motor speed as the thickness of thelubricating film increases. However, even as HF circulatingbearing currents themselves do increase, the detection ofthese currents through the RF based method is more difficult

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(a) Measured bearing current, bearing voltage, and RF signal.

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(b) Computed bearing impedance.

Fig. 9. Motor MA-15 and inverter IA-15, external HF voltage supplyof DE bearing: measured currents, voltages and RF signals and computedbearing impedance for 300 kHz HF supply voltage and 7Hz supply frequency(corresponding to 210 rpm); computed bearing capacitance during capacitivebehavior Cb = 0.21 nF.

with the NDE bearing shortened. The lack of contribution ofradiated energy from the NDE bearing towards the radiatedsignal may be interpreted as one factor contributing to thisobserved behavior.

C. External Supply of HF Bearing Currents

These tests were carried out to further understand the reasonfor the observed behavior. Emphasis was placed on operationat low rotational speed of a few hundred revolutions per minutewhere HF circulating bearing currents are more prevalent andelectrically insulating behavior of the bearing followed by adischarge occurring within the bearing is less expected. Notealso that even at 1MHz, the impedance provided by the 5mmthink electrically insulating layer is in the order of severalkiloohms which is at least by a factor of 103 larger than theone of the bearing current path. Thus, any current flow acrossthe NDE bearing is negligible.

1) Experimental Results with Motor MA-15: The bearingmay form a capacitive film even at low rotational speed.However, the capacitive behavior does not exist constantly butchanges to ohmic behavior. Over a certain time of some tensof microseconds, the impedance increases slightly, before thebearing impedance turns mainly capacitive again. Exemplarily,Figs. 9 and 10 show the measured DE bearing currents andvoltages, the detected RF signals, as well as the computedbearing impedances for 300 kHz HF supply voltage and 210as well as 300 rpm rotational speed.

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(b) Computed bearing impedance.

Fig. 10. Motor MA-15 and inverter IA-15, external HF voltage supply ofDE bearing: measured currents, voltages and RF signals and computed bearingimpedance for 300 kHz HF supply voltage and 10Hz supply frequency(corresponding to 300 rpm); computed bearing capacitance during capacitivebehavior Cb = 0.38 nF.

2) Experimental Results with Motor MB-15: Similar re-sults are obtained for the larger machine as for the smallermachine: Again, the bearing may form a capacitive film even atlow rotational speed. As in the case of the smaller machine, thecapacitive behavior does not exist constantly and also changesto ohmic behavior. In contrast to the case of the smallermachine, states with less pronounced behavior, i.e. capacitiveor resistive, in which phase angles in the order of -45° areobserved, too. Figs. 11 and 12 show again the measured DEbearing currents and voltages, the detected RF signals, as wellas the computed bearing impedances for 300 kHz HF supplyvoltage and 210 as well as 300 rpm rotational speed, now forthe larger machine.

D. Analysis

For certain short time intervals, a bearing is observed toform a capacitive film even at low rotational speed. In additionto this capacitive mode, the resistive mode expected fromthe conventional understanding occurs. The transitions arenot always instantaneous (when compared to the time scale,i.e. taking place within a few microseconds.) A change fromresistive to capacitive mode may for example be preceded bya slight increase in the bearing impedance before a rathersteep change to capacitive behavior occurs. The transitionsare significantly less distinct for the larger machine. Bearingtemperature as well as voltage applied across the bearing areexpected to influence the bearing impedance behavior, too.

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Fig. 11. Motor MB-75 and inverter IB-75, external HF voltage supplyof DE bearing: measured currents, voltages and RF signals and computedbearing impedance for 300 kHz HF supply voltage and 7Hz supply frequency(corresponding to 210 rpm); computed bearing capacitance during capacitivebehavior Cb = 1.7 nF.

Their analysis is subject of further research and not within thescope of this paper. While further research is required to fullyunderstand the observed behavior of the bearing impedance,the results show that some capacitive mode may exist themoment HF circulating bearing currents begin to flow thateventually allows such currents too to be detected through theRF based method.

V. CONCLUSIONS

Interpreting our observations in the context of HF cir-culating bearing currents, we postulate the following: For“sufficiently” large HF circulating bearing currents and energystored in the circuit before these currents start to flow, the oc-currence of these bearing currents, too, can to some extent bedetected using the proposed non-intrusive RF based method.The energy release during a very short time–translating intothe radiating power–occurs as the current–driven by the HFvoltage in the loop of the HF circulating bearing current–paths through the formerly electrically insulating lubricatingfilm of the bearing. This form of “penetration” brings alongsome energy conversion as well as dissipation that radiatesvia the machine acting as a transmitting antenna and canbe detected through the receiving antenna. Further researchis required to better understand the influence of bearingtemperature, machine load and rotational speed, as well asHF bearing current magnitude and frequency on the emissioncharacteristics.

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Fig. 12. Motor MB-75 and inverter IB-75, external HF voltage supply of DEbearing: measured currents, voltages and RF signals and computed bearingimpedance for 300 kHz HF supply voltage and 10Hz supply frequency(corresponding to 300 rpm); computed bearing capacitance during capacitivebehavior Cb = 8.5 nF.

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[2] S. Chen and T.A. Lipo, “Source of induction motor bearing currentscaused by PWM inverters,” IEEE Trans. En. Conv., vol. 11, no. 1, pp. 25-32, Jan./Feb. 1996.

[3] P. Link, “Minimizing electric bearing currents in ASD systems,” IEEEInd. App. Mag., vol. 5, no. 4, pp. 55-66, Jul./Aug. 1999.

[4] H.E. Boyanton and G. Hodges, “Bearing fluting,” IEEE Ind. App. Mag.,vol. 8, no. 5, pp. 53-57, Sep./Oct. 2002.

[5] R.F. Schiferl and M.J. Melfi, “Bearing current remediation options,”IEEE Ind. App. Mag., vol. 10, no. 4, pp. 40-50, Jul./Aug. 2004.

[6] A. Muetze and A. Binder, “Don’t lose your bearings - mitigationtechniques for bearing currents in inverter-supplied drive systems,” IEEEMag. Ind. App., vol. 12, no. 4, pp. 22-31, Jul./Aug. 2006.

[7] A. Muetze and A. Binder, “Practical rules for assessment of inverter-induced bearing currents in inverter-fed AC motors up to 500 kW,” IEEETrans. Ind. Electron., vol. 54, no. 3. pp. 1614-1622, June 2007.

[8] A. Muetze and A. Binder, “Techniques for measurement of parametersrelated to inverter-induced bearing currents,” IEEE Trans. Ind. App.,vol. 43, no. 5, pp. 1274-1283, Sep./Oct. 2007.

[9] V. Sarkimaki, Radio frequency method for detecting bearing currents ininduction motors, PhD Thesis, Lappeenranta University of Technology,Finland, 2009.

[10] J. Ahola, V. Sarkimaki, A. Muetze, and J.A. Tamminen, “Radio-frequency-based detection of electrical discharge machining bearingcurrents” IET Electric Power App., pp. 386-392, vol. 5, no. 4, Apr. 2011.

[11] A. Muetze, J. Tamminen, and J. Ahola, “Influence of motor operatingparameters on discharge bearing current activity,” IEEE Trans. Ind. App.,vol. 47, no. 4, pp. 1767-177, Jul./Aug. 2011.

[12] A. Muetze and A. Binder, “Calculation of circulating bearing currents inmachines of inverter-based drive systems,” IEEE Trans. Ind. Electron.,vol. 54, no. 2, pp. 932-938, Apr. 2007.

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